Mobile Navigation

Search form

You are here

sub-floor oven slab

sub-floor oven slab

So I'm working from the Tom Jaine book Building a Wood-Fired Oven and the plan calls simply for poured concrete, a layer of sand, and then the firebrick. I'm using 3" thick firebrick. I've seen some people recommend additives to the concrete (such as perlite) to give it some additional inulating quality. Is this necessary or overkill? It seems that between the sand and the firebrick, not much heat will permeate down through the slab as lost heat. Isn't the bigger concern to insulate the roof? The plans show 18" thickness from the interior wall surface to the exterior.

Incidentally, this oven is being build inside my house, which is a hundred years old and rather cool in the winter. A little heat loss to aid the house might not be a bad thing?

......with the Tom Jaine book, but as of late the emphasis has been on using as much insulation as possible under, on top of, and around your wood fired oven. I recently completed my Alan Scott style oven and built it according to the "Ovencrafters" plans. The "Scott" design uses a layer of vemiculite/portland cement mix under the concrete slab that supports the hearth bricks. I will make one modification and add more insulation under that layer of "Vermicrete".

Considering that you are building this oven in your home, I am sure that you've researched all building and insurance regulations that would be applicable to your area. I would love to have an oven in my home, but I'm not sure I would like all the additional worries and or building hassles that come with it.

You will get an amazing amount of heat bleed downward and outward if you do it this way; chewing fuel in the process and creating a rapidly cooling oven. The Scott method is a good one; I used it in my oven build ten years ago. But since then, I've added a two inch layer of high heat refractory insulation board under everything.

There is another method. First, a reinforced sub-slab is poured. On top of that, a "tray" of six inch block is built around the perimeter. The tray is filled with sand, then a two/four inch slab of reinforced concrete is poured over the sand, making an effective heat sink that's buffered from direct contact with the sub-slab. Personally, I prefer to place high heat board between the block and the sand, then insulate the bottom of everything (as above) once the oven has been in use several months (so you know it's very dry).

You can also place firebrick directly on top of insulation board, but this is really done when quick firing and lesser heat retention over time is the objective.

The hardware store says they can order a sheet of 3" insulation board. They say it will melt at 250F, and shrink at 180F. They didn't have a brand name. Is this the right stuff? If we go 3" firebrick floor, 1" sand, 4" concrete slab, 1" sand, how much heat would get down to the insulation board? The one time I worked with a brick oven, the exterior brick never got uncomfortably warm to a bare hand. I imagine setting the insulation board on top of a 1/2" wonderboard (cement w/ nylon mesh), and supported underneath with framing to pour the concrete slab.

The 3" board the hardware can get sounds like the foam board you would insulate the outside of a house with. I think you should be looking for ceramic fiber board and I don't think local hardware stores would carry it. I was able to get mine through work, perhaps CanuckJim will chime in and let you know where he got his. Not sure you would need three inches of the stuff.

My oven's hearth is 4.5" of brick on top of 3" of reinforced concrete. This all sets on top of 2.75" of vermiculite/portland cement mixture. I still lose heat through the hearth. I will be installing my insulation board in the spring.

Hi i really am stumped every time i find a project to do my small little town never has what i need. I have the Forno Bravo plans to make a pompeii style oven and i dont want to lose any heat but i want it to be cost efficient to build. you mention high heat refractory board is this another name for FB board? In the plans it says to do a concrete hearth with vermiculite/concrete on top or do the concrete hearth with 2" of FB board can i do all three if i can find the FB board

In my experience, a lot of this seems to be brand-name marketing without performance justification. If you're doing a dome-style pizza oven, you might want a continuous fire even after you get the oven up to heat. So retaining heat is not the same concern as if you had swept the fire out. In that case a 4inch subfloor with 1inch sand and 2inch firebrick would be more thermal mass than you'd be likely to saturate with a two or thee hour fire before cooking. So it would seem to me at least that all this fuss is not worth the time and money, especially if it isn't going to be commercial.

In addition to what CJ says, less insulation (even below the oven floor) means more wood to heat the thing and shorter effective baking times. You can neve have too much insulation.

Also, +1 on following to the letter all applicable codes and, in addition, pull a permit and get it inspected. Otherwise, if there is an accident, you may find yourself uninsured and you may have quite a hassle if you ever decide to sell your house.

It can be easy to over-generalize the house insulation rule of thumb: "heat rises". That rule of thumb is definitely applicable to things filled with air (which houses of course are). For solid constructions though it isn't a very good guide.

You really, really don't want the hardware store stuff; it will melt, especially if you have an ash/coal bucket underneath it. Besides, you really don't want to inhale the fumes of melting PVC. You need a two inch layer of K-Fac 19, the refractory industry standard for such applications. It's relatively soft and is not meant for load bearing. The denser load-bearing stuff goes under various names, including SuperIsol. Check out Chicago Firebrick on the web for a dealer near you. Alternately, contact pottery makers/clubs to find out where they get their materials. K-Fac comes in a 50 sq. ft. box of, I think, 2'x12" panels. Use the leftovers for your heat plug door.

This board should not be applied to the bottom until the oven has been in use for a month or so (very, very dry); otherwise, you could trap moisture from the slab. Not good, because it will turn to steam and cause cracking.

You do not want to pour on Wonderboard that will stay in position later. Again, you'll be trapping moisture from the slab. Make a form out of 3/4" ply, supported by 2x4s. Once the slab is set, remove the form so everything dries evenly. Once the oven is dry, dry, dry, you can add the K-Fac, supported by steel studs just enough to hold it tight to the bottom with no air gaps. It should not be used structurally.

I'm not at all sure I understand the whys and wherefores of the sandwich approach you're taking, but to answer your question in this scenario, quite a bit of heat would migrate downward. The oven you worked with was most likely well built and well insulated. It's really a matter of care and procedure.

I've already outlined the various methods, and I'd suggest you take a long, hard look at this before proceeding.

I just talked to the thermal-smart guy at Chicago Firebrick and he told me:

If the 3" firebrick surface is 500F, the other side of the firebrick will be close to 400F. Sand is the best cost-efficient insulator, so firebricks sit on top of sand. He wasn't sure if 2" of sand is 50% better than 1" or what the dimisishing return would be. To me the issue is stability of the sand, whether the brick floor might shift more on a deeper bed of sand. He thought that with 4" of concrete there would be less than 150F radiating out the bottom once the oven settled into baking range. He recommended their ceramic paper, 1/8" between the sand and concrete. But at $1.25/sq.ft. with a 250 sq.ft. minimum, that seems counterproductive. He also felt that any insulation benefit from vermiculite additives in the concrete would be negligible.

Here's a new thought:

I live in an orchard town, and the most available/cheapest wood is fruitwood pruning. I've heard some here say that if you use such wood, you burn 3x as much to achieve the same as if you burned 1/3 as much kiln-dried. So if I store fruitwood under the oven and use the bleed-out heat to cure the wood, not so much worry?

Further, what about putting some sort of blow torch on a natural gas line (or a propane rocket heater) to reheat after the first burn has bled off? Or is it not such a big deal to just build another fire?

I agree pretty much with what CJ said. (Exception is that wonderboard won't really trap moisture any more than the concrete itself would. Heck, it IS basically concrete.) The goal you should be striving for is an insulated envelope with sufficient mass on the inside (trade off here -- more mass = more stabile temps, but more wood to heat) and sufficient insulation on the outside to hold the heat in.

As for how well any particular insulation works, well that's a tough call. Take sand for example. Sure, it doesn't get as hot to the touch as the firebrick, but in a WFO, I'd classify sand as part of the thermal mass, not insulation. Or take wood. You can safely use wood for a pot handle since it won't get hot enough to burn your hand, so you might be tempted to call wood a good insulator. But I wouldn't rely on the wood siding of my house to keep me warm in the winter! Another example would be sheets of bubble wrap with reflective coatings on both sides. That stuff is touted to have huge R values, and under some conditions I'm sure it does. But it does almost nothing to keep a house warm.

The only real way to guage the effectiveness of a particular insulating scheme is to build it and carefully measure heat loss over time under real world conditions. Since that's been done by generations of prior builders, you're probably better off not reinventing the wheel here unless you want to drop a bundle of cash and build a bunch of ovens in the interest of WFO science. ;-) I suppose the ideal WFO would be 8"-10" of dense masonry surrounded by a mirrored vacuum chamber, like a brick in a big thermos bottle. But since that's not really practical, you go with what's at hand and with what's been demonstrated to work over time.

CJ's techniques are interesting, and he uses some pretty modern materials not available at the local lumber yard. Are those methods sufficiently "better" than the traditional methods to justify their use? For him they are. For you, only you can tell. Wanna know for sure? Get a firebrick, install a thermocouple in its center, heat it up to 500° and wrap it in whatever insulations in whatever thicknesses you are thinking of using. Measure the heat drop over time and compare them.

My experience is that a lot of the "experts" you might be tempted to seek advice from can only answer your questions from their own industrial perspective. The Chicago firebrick guy is probably basing his answer on his knowledge of the industrial kilns or ovens that his customers have and that have very different uses and requirements than your WFO. Something that makes a good glassmaking furnace won't necessarily make a good WFO for bread. (While it maybe great for the Chicago guy's customers uses, 150° surface temps over the size of the oven floor is a HUGE loss for a WFO! The outside of my oven floor, constructed with traditional vermiculite/portland insulation, doesn't get to 10° above ambient.) Same deal for fireplace experts who often fancy themselves qualified to pontificate on WFO construction. Things that make a good fireplace probably won't make a good WFO. So I'd advise you to pretty much ignore any advice from a non-WFO expert.

As for using "waste" oven heat to dry wood, research detailed in Alan Scott's book shows that drying wood with oven heat is actually counterproductive. You use more heat drying the wood than you gain by later burning the dried wood. (This assumes that the oven is in daily use. IME, throwing a full load of wood into a hot oven that you don't intend to use again for several days is a very good idea, since the heat in that case would otherwise truely be wasted.) IME, fruitwood is a fine fuel. Just don't burn it green. I split wood for WFO use to almost kindling size and then dry it well for another 6 months or so if possible. The fruitwood I usually burn is cherry and, although it takes a bit more than, say, oak, it's nowhere near 3X. Maybe 25% more by weight. Also, you want a wood that burns down to coals and ash quickly. Fruitwoods are good for this. (Tho' my favorite wood is maple.)

But of course, assuming fire-out baking, the bottom line is how much it costs to bring the oven to temp, so wood that costs $1 to get the oven to temp is better than wood that costs $1.50, irrespective of the relative volumes of the two types of wood. After all, a 500° brick is a 500° brick -- it makes no difference to the bake if you heated it with scrap pallet wood or by burning $1,000 bills.

As for heating with a propane salamander -- they don't get hot enough to do you any good. I'd be surprised if they didn't actually lower the oven temp since they introduce a pretty significant air stream. I have seen gas burners for ovens that can be used to "top off". I believe Alan Scott's book mentions them. I have no idea where you might find such a beastie, but I doubt it would be worth the effort for a non-commercial oven. A short second (or third) burn is pretty easy -- I often do this, especially when I'm baking a variety of things sequentially. The secondary burns are pretty quick, since the oven's already preheated.

Sand, in fact, is a pretty crummy insulator, unless you live in Sicily in 1920 and it's free. This is especially true in the face of the truly great insulators available out there now. Surprised you got this kind of advice from CFB. It might be worth finding out whether your source has any experience with WFOs; doesn't sound so. We're talking about retained heat here, so you need something to retain it and something to keep it retained. Bedding the bricks in a deep layer of sand is a poor idea; the hearth would never stay level. Why you would consider it, even one inch, in the first place is curious. Most people use fireclay and brick sand hydrated to the consitency of peanut butter, spread over the slab with a notched trowel so the bricks can be tapped level.

The kind of gas burner you'd need to do what you want costs upwards of $3K.

I don't use vermiculite/Portland, rather the castable insulator Matrilite 18, as I've said before. Many, many people have used the former, though, and gotten decent, not great results. The fact of the matter is that either one of these is used as a separate insulating layer, not mixed into the slab pour. The K-Fac is added below the insulating layer. Sounds like you've got a lot more research to do before beginning your build.

Can't imagine a scenario where any sort of genuine refractory insulator would be counterproductive in a WFO. If you insist on an uninsulated slab, better get out the chainsaw and attack the nearest orchard, cause you'll need it all to bring that baby up to speed and keep it there. After that, consider the furniture.

We're really talking about efficient fuel use and retaining as much heat as possible for as long as possible. The direction you're going will yield neither one.

Well CJ, I was trying to take you seriously. I tried to say "SuperIsol, okay, I should look into that." So I did. I tried to say, "Chicago Firebrick, I should give them a call." So I did. And tell me, what sort of thermal engineering training do you have, that the engineer at Chicago Firebrick doesn't have?

When you say over and over, "Well you'd have to be stupid to take that approach" it might make your homies here think you're wonderful, but it doesn't always endear you to strangers.

Yes, I've got research to do as I plow through this project. I think it is probably apparent that this is not my first stop on that journey. But this is my last stop here. I don't think I need any more CJ arrogance.

I'm a journeyman mason myself, and I'm working with a master mason. I think we might know a thing or two between us. We'll probably talk to a few other folks as well. In all things, a grain of salt.

This is my two cents. I just got done with my oven last fall, It's functional at this point but I still need to dress up the outside and put it under permanent cover.

I looked at the entire post and I didn't see where anyone called you stupid.

From my experience I will tell you that insulating under the hearth is extremely important. I have 3.5 inches of vermiculite/Portland cement mix topped with the fire clay and sand mix to set my floor.

With that said I wish I insulated more under the oven. I wish I used some of the ceramic insulation board. The boys at my local refractory store could tell me all day long the specks of the materials but did not have a clue as to it's use in a WFO.

I will tell you from my personal experience that the advice that you have received in this thread is GOLD. If you choose to ignore the advice that is your choice. Plow - On

I've never called anybody stupid on this forum or any other. I think "arrogance" is being mistaken for experience. I've been building and installing WFOs for eleven years now, and I've repaired far too many for people (residential and commercial) trying to reinvent or circumvent designs, cut corners for false economy. My reputation in this field is international; I've given wood fired baking courses to hundreds of people from around the world; I've written a well-regarded book on WFO bread baking, WFO history, oven types and construction; and acted as a consultant on many, many WFO builds. I guess that answers your question, because all these things qualify me to respond based on what specific things work in a WFO and which ones don't, or at least what is less than efficient or could cause problems down the line. I've been trying to give you the benefit of all this--information I usually get paid to give--but it occurred to me that maybe it was being ignored. So be it. Perhaps the value of it, as Faith says, will be picked up by others contemplating a build. Good.

It's worth mentioning that I have taught six conventional masons the hows and whys of these ovens.

Such a build presents a unique set of circumstances that take knowledge and repetition to master. Why would I take the time and bother responding at all unless I was trying to help you out and guide you in proven directions. Ego building? Don't think so. The only real difference between me and the late Alan Scott is that I choose to take advantage of new refractory developments, while he did not. This is not to put me on the same level as Scott; I'm merely following in his pioneering footsteps.

For the last five years I've worked with a refractory supplier who is very conversant with kilns, foundaries, glass making, but who at first knew nothing about the construction and requirements of these ovens. I, on the other hand, at first knew nothing of the wide range of superior materials available. Working together over time, we've pooled our separate areas of expertise to come up with a very workable and efficient system, one which he sells to individual builders and one which I recommend and use myself. I rely on him; he relies on me.

I wanted to put you in touch with CFB to determine where the materials mentioned could be bought, not to solicit a separate opinion from, apparently, someone who knows a lot about refractory specs but not very much about WFOs. I think my refractory supplier would back me up on that supposition, because five years ago he was in exactly the same spot.

In the end, I was not calling you anything, other than inexperienced. A grain of salt indeed.

or "partition tile" is what that product is generically called. Used extensively in construction prior to the wide-scale use of cement block (AKA CMUs). Not sure how well it would work for a WFO floor, but a word of caution -- they crack pretty easily and, once cracked, they have almost no structural integrity.

So, I finished building the oven, and I used a 4 inch concrete slab. Concrete, not vermicrete. I put an inch of sand on the concrete, then laid 3 inch firebrick. The best guess on the firebrick is that it is 65% aluminum silicate? A guy at the refractory shop said as much. The roof turned out to be 9 inches of the same firebrick. On top of the roof firebrick I did a vermiculite stucco per formulas found elsewhere here. Then I plastered over that. I insulated between the sidewalls and the facade with vermiculite.

The results? When I burned for a pizza party today the interior masonry temp on the roof got above 700F, as did the floor. The exterior temp on the roof maxed at 150F-- not bad for no insulation. With such a low exterior temperature I think I could probably just add some fiberglass batts (not an expensive blanket) in the R19 range. As for the slab, the temperature underneath never got more than 100F.

When deciding to do concrete or vermicrete I talked to a couple of different refractory industry specialists. They both said that vermicrete is a waste of time, and that it compromises the strength of the concrete under a load. My results seem to confirm this.

We baked 9 pizzas today, and finished with four sourdough loaves and four baguettes. I still have a lot to learn about how to manage the fire, but overall I think I'm pleased.

...was about three hours. The oven is pretty big, 30 sq. ft. hearth, so I block off half of it with a false wall of firebrick when doing pizza and small loads of bread. I think that the mass of the roof, 9 inch thick, probably takes more time to saturate, but should hold more heat longer?

I didn't cure it much at all. I had a feeling the sub-floor wouldn't get as much heat as people were saying. The floor cured two months before it took heat from the fire. As for the roof, it is made of bricks that came with a key-cut shape, so there is minimal mortar. In fact, I only put mortar on two of four sides, and laid them dry across the ranks. As a result, I had to do three layers of stucco and plaster to seal it all in. I think there might be some heat leak on the dry joints, but overall, it doesn't seem to be terrible. Minor insulation should fix it.

I got a screamin deal on the firebrick: $0.50/each! And they're much bigger than regular 4x8 bricks. My hearth bricks were 3x6x13 (2.5 x larger than conventional) and I still only paid .50 cents ea. But boy are they a bugger to cut!

No photos yet, its nothing special. When I get the heat figured out, maybe I'll post up some crumb shots. Mostly I just wanted to say, there's more than one way of doing these projects. When someone says, "Its gotta be like "X" or it just won't work" then I get a little suspicious. We all have good ideas and varying amounts of life lessons learned.

you don't necessarily "control" it, but rather monitor it. You heat the oven past your desired temperature,, then sweep out the fire and coals. As the temperature cools through your target window, you bake.

All original site content copyright 2015 The Fresh Loaf unless stated otherwise. Content posted by community members is their own. The Fresh Loaf is not responsible for community member content. If you see anything inappropriate on the site or have any questions, contact me at floydm at thefreshloaf dot com. This site is powered by Drupal and Mollom. It is part of the Federated Media Publishing - Food channel. Click here for advertising information.